The invention relates to the field of radio communication and, in particular, to the reduction of interference in signals coupled from a transmission antenna into a local receive antenna in the presence of a local multipath.
In co-site environments, a collocated source usually interferes with the receiver due to the finite isolation between the transmit and receive antennas. This interference in a co-site environment is a combination of several factors. There are two principal reasons for interference in a co-site environment: (1) desensitization caused by one or more nearby high-power transmitter carriers, and (2) wideband moderate to low-power interference components associated with those carriers. These interference components are received by the collocated radio and degrade system operation.
Wideband interference for each transmitter consists of three components: (1) near-in noise at frequencies close to the carrier, (2) periodic spurious signals across a major portion of the adjacent band, and (3) a broadband noise floor also spanning a major portion of the adjacent band.
Fixed narrowband filters are applied to transmitters on an individual frequency channel to reduce spurious and broadband noise interference but are not applicable for radios which continually change channels. Frequency hopping (FH) filter technology has been applied to improve system performance in such systems. This technology reduces co-site interference through a front-end applique that limits the bandwidth of the wideband interference transmitted to the collocated receiver.
With frequency hopped waveforms, a look-ahead technique can protect the receiver at the expense of system bit-error-rate (BER). When control logic senses that one or more of the upcoming transmit frequencies will cause desensitization, the transmit carriers are internally terminated or attenuated. In an alternative approach, the collocated receiver is disabled when desensitization is expected. Performance improvement depends on the hop-set frequency range and is related to the bandwidth of the implemented filter. Larger hop-set bandwidths ( greater than 1000 times channel bandwidth) permit more improvement for a given filter bandwidth because frequency conflicts occur less often. This technique, however, may seriously degrade system-level bit error rates because transmission and/or reception are periodically disabled or attenuated.
An Interference cancellation system is based on amplitude and phase adjustments of a sample of the collocated transmitter, which is then used to cancel the radiated interference at the input to the protected receiver. The adjustments are made by a correlation-based adaptive controller using feedback derived after the cancellation process.
Carrier cancellation with an ICS can range from 40 to 50 dB, totally eliminating desensitization without disabling the transmitter or receiver. However, dispersion and multipath effects can seriously limit cancellation of the wideband interference associated with the carrier. Dispersion is also time-varying, particularly in on-the-move embodiments. Therefore, previous ICS based co-site solutions have had limited performance for dispersion and multipath.
Current methods of dealing with dispersion phenomena have been confined to compensation by hop-set restrictions, which may permit static calibration techniques to pseudo-optimize performance over limited bandwidths. Static calibration techniques have been limited to the addition of time delay within selected branches of the ICS. Experimental measurements on a two-antenna mobile relay configuration at VHF have confirmed the presence of virtual time delay shifts, spectral amplitude tilts, and both quadratic amplitude and phase errors. No prior art has considered a generic solution to the entire problem of dispersion-induced performance limitations.
The invention is an interference cancellation system for improved rejection of a signal coupled from a transmission antenna into a local receive antenna in the presence of local multipath. This interference cancellation system includes a frequency hopped radio adapted to operate in a dispersion environment. The frequency hopped radio has an instantaneously limited bandwidth communication channel operating over a substantially larger bandwidth. Transmission of a signal from the frequency hopped radio is via a directly coupled reference path and a received coupled path. A delay means is used for variably adjusting the phase slope delay of the signal in one path relative to the other path. The system employs an amplitude slope means for variably adjusting the amplitude slope of the signal in one path relative to the other path. A digital controlling means, associated with the delay means and the amplitude slope means, provides improved matching of the signal in each path, thereby enhancing the performance of the co-site interference reduction system.
Therefore, it is an aspect of the invention to provide a wideband co-site interference reduction system that uses an integrated FH transmit filter and an interference cancellation system (ICS), optimally combined to maximize wideband co-site interference rejection.
It is another aspect of the invention to provide an interference cancellation system that is applicable to frequency-hopped-waveform relay architectures and is extendable to multiple-antenna non-relay instrumented installations.
It is another aspect of the invention to provide an interference cancellation system having an applique architecture for ease of installation for both on-the-move and stationary platforms.
Another aspect of the invention is to provide an interference cancellation system having automatic dynamic equalization of dispersion components such as time delay and spectral amplitude tilt to enable wideband ICS nulling of the collocated transmitter. Operation can also be extended to other forms of dispersion, such as quadratic amplitude and phase errors.
It is still another aspect of the invention to provide an interference cancellation system which has an improved ICS adaptive controller based on a limiter or AGC-modified least-mean-square (LMS) algorithm, including off-line low-power FH filters to minimize correlation on extraneous signals.
It is still another aspect of the invention to provide an interference cancellation system which uses a transient suppressor to blank the receiver input and eliminate AGC capture during ICS transmitter nulling.
Finally, it is still another aspect of the invention to provide a wideband interference cancellation system using component designs supporting high power operation for maximum distortion-free range extension.